Over the weekend, one of my sites has "strong" tags inserted everywhere. I checked the templates, they are the same. Any idea how they could have gotten in there?
It sounds like you have some content wackery (just made that word up, I'm proud of myself!) going on. The content that gets rendered inside of a channel entries loop lives inside of a database table called
exp_channel_data. When you call a channel entries loop, the rendered content comes from the database, and this is content that is generally editable by users who maintain your site.
Most likely what happened is someone went in to their control panel, and erroneously added
<strong> tags in a bunch of places. Look over your channel entries to see if it's happened in more than one place. If you have a site with a lot of pages, you can search for instances of this tag with a MySQL query:
SELECT * FROM my_schema.exp_channel_data WHERE field_id_666 LIKE "%<strong>%";
field_id_# to whatever your main content field id is (usually a field that gets called "Body" or "Main Content"), and you can do bigger searches to look for channel entries. If you have a small amount of entries, it's probably easiest to just use the Control Panel to investigate this data.